A system to detect fissile material within a container is disclosed in U.S. Pat. No. 6,768,421, the contents of which are incorporated herein by reference. Here, a spreader has been adapted to include detection apparatus for detecting radioactive material or potentially radioactive shielding material within the container.
This type of arrangement has several drawbacks. Firstly, to maintain control of the spreader, and still operate as quickly as possible, spreaders can be complex devices. This complexity suggests a conflict of purpose, whereby the ideal location for the detection equipment will be compromised. Alternatively, such a spreader will need to be substantially modified in order to accommodate the detection system, leading to costly redesign. Secondly, maintenance of the detection equipment will lead to downtime of the spreader. Whilst it would be preferable to investigate every container, it is still necessary to maintain the flow rate of containers through a port. It follows that downtime of a spreader can lead to a significant delay and consequent financial loss whilst the spreader is unavailable, not for any in the spreader, but due to a fault with the detection system.
An alternative arrangement is shown in U.S. Pat. No. 6,998,617, the contents of which are incorporated herein by reference. Here the detection equipment is mounted to a frame which is placed intermediate the spreader and the container. To ensure a correct alignment position of the detection equipment, the frame is mounted to the container using twist-locks on bottom legs of each corner of the frame, with the spreader mounted to these legs in an upper portion, again, using twist-locks. Thus the spreader is indirectly mounted to the container through the intermediate detection frame.
The length of the connection between a spreader and an engaged container is factored into the design of the spreader so as to avoid damage to the spreader and container should an unforeseen force be applied. The use of an intermediate frame between the spreader and container will increase this length, and consequently increase the lever arm associated with this connection, by up to factor of 2.
By doubling the length, the applied moment will also double, increasing the chance of damaging the twist-lock connection. Further, the “flipper” at each corner of the spreader, which is designed to apply a specified load in order to locate the spreader onto the container, relies upon the lever arm between the point of rotation and the engagement corner of the container. By doubling the connection length, the lever arm is also doubled and therefore halving the applied force of the flipper used to locate the spreader upon the container, thus reducing the effectiveness of the flipper.
In any event, in order to use a flipper upon a container with the intermediate detection frame in place requires a modification of the spreader to use a longer flipper. This returns, of course, to the original problem of having to modify the spreader, albeit only cosmetically. Further, a spreader having modified flippers cannot easily be used upon a container without the intermediate frame being present.